In the fabrication of semiconductor wafers used in making a variety of semiconductor circuit devices, the importance of minimizing contamination on the wafers has been recognized since the early days of the industry. However, as the end product devices have become more and more miniaturized and complex, the cleanliness requirements have become increasingly more stringent so that the devices will function properly. With the reduced size of the devices, a contaminant occupies an increased percentage of the available space for current elements, and hence cleanliness of the materials becomes far more critical.
As the devices become more complex, they also become more valuable, such that unsatisfactory products represent a very significant loss of revenue. A cassette load of large diameter wafers may have an end process value of as much as a million dollars. Also, there is the cost incident to unusable end products that might arise as a result of the discovery of unsatisfactory semiconductor devices after their combination with other components.
In addition to the foregoing cost, there are the major expenses associated with the cleaning processes themselves. One of the major capital expenditure is in the cost of cleaning and drying equipment and associated plumbing, heating and cooling equipment, robotic wafer handling apparatus, computerized control equipment, apparatus for storing and disposing cleaning solutions, and the clean room space required for the apparatus. Of course, there is the cost of the cleaning solutions and the cost of heating, cooling and filtering the solutions, as well as the cost of storing and disposing of them. In view of environmental concerns and regulations, the cost of disposal of certain materials can be greater than the cost of the material being discarded.
Still the most common system for cleaning semiconductor wafers utilizes a series of tanks containing the necessary cleaning solutions, with the tanks being positioned in a "wet bench" in a clean room. A batch of semiconductor wafers is moved in sequence through the series of tanks, usually by means of computer controlled automated apparatus. A major concern with this type system is that of contamination occurring as the batch of wafers is transferred from one tank: to another. Also of significance is the possible contamination introduced by the handling apparatus itself. Further, whenever wafers are moved, there is the risk of damage to the wafers due to mishandling.
Another system, rather than utilizing tanks which are open to the surrounding clean room, utilizes a full, continuous flow of cleaning solutions through a pipe-like construction. A supposed advantage of that system is that by keeping the wafers immersed in cleaning fluids throughout the process, the risk of contaminants being on the wafers is decreased. The effectiveness of this system, however, is somewhat controversial, and the apparatus is relatively expensive to purchase and to operate.
Still the most commonly used cleaning solutions are those developed by RCA many years ago employing hydrogen peroxide chemistry, particularly those referred to as "standard clean 1" or "SC-1" and "standard clean 2" or "SC-2." SC-1 typically comprises ammonium hydroxide, hydrogen peroxide and deionized water in the following ratios: 1 NH.sub.4 OH: 1 H.sub.2 O.sub.2 : 5 H.sub.2 O. SC-2 usually comprises 6 H.sub.2 O: 1 H.sub.2 O.sub.2 : 1 HCI. Typically wafers are immersed in these solutions for 10 minutes at 25.degree.-80.degree. C. for each solution. Intermediate and final rinses of deionized water are used between chemical steps. If the wafers are particularly contaminated, there is an initial cleaning step utilizing a solution known as "Caros" or "Pirhana," typically comprised of H.sub.2 SO.sub.4 and H.sub.2 O.sub.2 in ratios varying from 2-5:1. Following the use of Pirhana there is frequently an additional etching step employing DHF (dilute hydrofluoric acid).
While those solutions contain the most commonly used, chemicals and those are the most common ratios, solutions with other ingredients and solutions with different ratios have been utilized, including some with relatively dilute solutions of the active ingredients. A modified SC-1 mixture of 0.01 NH.sub.4 OH:1H.sub.2 O.sub.2 :5H.sub.2 O has been reported to help reduce surface roughening.
An additional technique for loosening particles, is that referred to as megasonic cleaning. In this technique, highly effective non-contact scrubbing action on both front and back side surfaces of the wafers is achieved by extremely high-frequency sonic energy, while the wafers are submerged in liquid. By utilizing the megasonic system, with standard cleaning solutions, films and adsorbed contaminants are removed at the same time that particles are being removed by the megasonic energy. Sonic waves of 850-900 kHz are generated by an array of piezoelectric transducers. Particles ranging in size from several micrometers down to about 0.2 micrometers have been efficiently removed with input power densities of 25 watts/in. Megasonic cleaning systems are available from VERTEQ, INC., assignee of the present invention.
As noted above, because of the advances in the miniaturization and functions of semiconductor circuit devices, improved semiconductor cleaning techniques are highly desirable. Some of the goals or industry needs are to reduce particulate levels to less than 0.1 micron, to reduce defect density levels to less than 0.001 particles/cm.sup.2, and to reduce surface metallic contamination levels to 1E.sup.8 atoms/cm.sup.2. In addition, it is desirable to eliminate chemical cross-contamination from transfer of cassettes from one tank to another, as with traditional systems. Further, an important goal is to control the cleaning processes to prevent or minimize surface microroughness of the finished product. Another goal is to reduce the high cost of ownership associated with wet chemistry processing, which includes the cost of cleaning solutions and their disposal, and many other elements. It is, of course, always desirable to lower the initial cost of equipment and to improve the reliability of the equipment.